Of the many instruments proposed for measuring surface reflectance optically, none are completely satisfactory with respect to cost, size, portability and accuracy. Most, if not all, prior instruments require an optical system that includes many lenses and other costly components such as half-silvered mirrors, monochromatic light sources, and special alignment schemes which increase cost, size, bulk, and reduce portability. For example, one prior instrument is called a Gloss meter or total integrated scatter instrument. In this instrument, light from an incandescent bulb passes through columnating lenses and is focused on the specimen at an oblique angle. The light is reflected away from the surface at an oblique angle into an optical system comprising additional lenses and a half-silvered mirror which reflects a portion of the light to the first photosensitive element and allows the rest to pass on to a second photosensitive element. Such an instrument requires a multitude of lenses that increase its size, bulk and cost. Moreover, the accuracy of the measurement depends strongly upon wavelength of light in a known relation to surface reflectance, the instrument's ability to separate between specular and diffused reflectivity components of the scattered light, the quality of the lenses and the relative alignment of the components. To overcome these disadvantages, it is a primary object to provide an accurate and highly effective and portable instrument of reduced size, complexity and bulk, for measuring surface reflectance. A further object is to provide a surface reflectance measuring instrument with component parts constructed and arranged to inherently produce an output that represents surface reflectance accurately and wherein certain ambient or environmental factors cancel each other out. A further object is to provide a new method by which the ratio of the light reflected from a specimen is compared with light of a known magnitude reflected from a similar specimen under the same illumination conditions by means of a reference photosensor with a provision through which the output of the instrument will be largely independent of the optical power coupled onto the surface. A further object is to provide an improved instrument of the type described which lends itself to the production of an inexpensive hand held instrument of small size as well as to the production of stationary or motorized scanning instruments or the production of instruments that will provide an output such as an alarm indicating whether the specimen meets expected reflectance criteria.
These and other more detailed and more specific objects of the invention will be apparent in view of the accompanying detailed description setting forth by way of example a few of the various ways within the scope of the invention these objectives can be accomplished.